Liam -
So many questions! Here are a few comments, FWIW:

You can use cherries instead of the other berries, but I think I'd
keep the bananas, figs and dates. I think they contribute some
flavors to the port that complement the berries, so I'd keep them and
use 18 lbs (or perhaps more) cherries in place of the other berries.

Worry about whether to add a bit of glycerin after you've finished the
wine and are about ready to bottle.

It may well be true that cherries are high in malic acid, but I'm not
sure why that would be a problem. Presumably you are not going to put
this wine through MLF. You could certainly use some proportion of
other fruit, which might increase the citric acid relative to the
malic, but then it wouldn't be as focused on the cherry. I don't see
why you'd bother, if what you are aiming for is something that is
distinctly cherry-flavored.

I think your calculations are correct. There is a tool called the
Pearson Square that is intended to simplify this type of calculation.
Personally, I find using a bit of high-school algebra works just as
well, and is (to me) less confusing.

Yes, you could use bourbon instead of Everclear. I usually use high-
proof rum (e.g., Bacardi 151) as I can't buy Everclear in Minnesota.
Using higher-proof spirits cuts down substantially on the amount you
need to add (and dilution of the wine flavors). No, in my opinion
there is no sense in using premium bourbon for something like this --
I'd lean toward Heaven Hill or something similar - drinkable but
inexpensive. With all the other flavors in there, plus some sugar,
you'd never be able to taste the difference. At least, I'm sure I
couldn't.

I can't give you any insights on the chocolate cherry stuff. I've
read posts from folks who have done it and seemed very pleased.
Personally, I've always thought that chocolate and fermentation didn't
really go together very well. There are (were) kits available for
making Chocolate Orange Port (RJ Spagnols) and Chocolate Raspberry
Port (WinExpert) that have pretty much received rave reviews -- you
might want to see if you can find one of them, they'd be a lot more
fool-proof.

Happy fermenting --

Doug